Generally, a plastic molding device for a semiconductor element has an upper platen for holding an upper metal mold, a lower platen as a base, and a movable platen for holding a lower metal mold. The movable platen is raised to mold-clamp the upper and the lower metal molds, and a lead frame and a semiconductor element placed between the molds are plastic molded.
FIGS. 1 and 1a show a prior art plastic molding device for a semiconductor element. An upper platen is provided for holding an upper metal mold 8. A tie bar 3 is provided between the upper platen 1 and the lower platen 2. There is also provided a movable platen 4 between the upper platen 1 and the lower platen 2. In order to conduct mold clamping the movable platen 4 is driven upwardly by a main oil pressure piston 5 and auxiliary oil pressure pistons 6, and the lower metal mold 9 fixed on the movable platen 4 is pushed towards the upper metal mold 8. Then, a plastic molding tablet 31 is inserted into the table insertion aperture 30 provided in plurality inside the lower metal mold 9, and there are placed a lead frame and a semiconductor element on the lower metal mold 9. A power unit 7 including a motor and an oil pressure tank is provided for supplying oil pressure to cylinders each containing the main oil pressure piston 5 and the auxiliary oil pressure piston 6.
After the mold clamping is conducted the press pressure is held at a predetermined pressure, and the plunger 32 provided at the lower metal mold 9 is driven to rise up by an oil pressure so as to inject plastics into the metal mold. After the semiconductor element is plastic molded by the injected plastics, the movable platen 4 is lowered so as to separate the upper metal mold 8 and the lower metal mold 9, and the plastic molded lead frame is taken out.
In this prior art device the plunger 32 is driven by oil pressure, whereby it is difficult to adjust the speed of the plunger 32 and the press pressure at high precision, resulting in difficulty in a stable plastic molding, additionally, the maintenance of the device becomes quite troublesome because inspection for oil leakage and exchange of sealing member are required. Furthermore, the device including the power unit occupies a large space, resulting in an increased cost.
Another prior art plastic molding device for a semiconductor element is disclosed in Japanese Utility Model Laid-open Publication No. Sho. 59-9538. In this device a DC servomotor is used for driving a spindle for plastics injection, wherein the spindle is provided with a plunger.